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Geologic Provinces of the United States: Pacific

Pacific - Cascades volcanic province

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Explore: Sierra Nevada | Cascade Volcanoes | North Cascades

The deep wilderness and rugged peaks of the North Cascade Range are only a few hours' drive from the metropoli of Washington and British Columbia. The average visitor to the range experiences a bit of Alaska and the Alps for the price of a weekend trip. The spectacular scenery of these mountains is carved from equally spectacular geology.

A mountain mosaic

The North Cascade Range in Washington State is part of the American Cordillera, a mighty mountain chain stretching more than 12,000 miles from Tierra del Fuego to the Alaskan Peninsula. Although only a small part of the Cordillera, mile for mile, the North Cascade Range is steeper and wetter than most other ranges in the conterminous United States.

Rugged topography of the North Cascades
Studying rocks can be very hard work. A geologist rests on Mount Sefrit with a view of Mount Shuksan, North Cascades National Park. Photo by R. Tabor, USGS.

In alpine scenery and geology, the range has more in common with the coast ranges of British Columbia and Alaska than it does with its Cordilleran cousins in the dry Rocky Mountains or benign Sierra Nevada. Much of the range is roadless wilderness preserved from commercial exploitation by inclusion in North Cascades National Park, the Ross Lake and Lake Chelan National Recreational Areas, and several dedicated wilderness areas managed by the U.S. Forest Service.

Rocks of the North Cascades record at least 400 million years of Earth history. The range is a geologic mosaic made up of volcanic island arcs, deep ocean sediments, basaltic ocean floor, parts of old continents, submarine fans, and even pieces of the deep subcrustal mantle of the earth. The disparate pieces of the North Cascade mosaic were born far from one another but subsequently drifted together, carried along by the ever-moving conveyer belt of tectonic plates that make up the Earth's outer shell. Over time, the moving plates eventually beached the various pieces of the mosaic at a place we now call western Washington.

As if this mosaic of unrelated pieces were not complex enough, some of the assembled pieces were uplifted, eroded by streams, and then locally buried in their own eroded debris; other pieces were forced deep into the Earth to be heated and squeezed, almost beyond recognition, and then raised again to our view.

Not so long ago

About 35 million years ago the Cascade Volcanoes grew across this complex mosaic of old terranes. Volcanoes erupted to cover the older rocks with lava and ash. Large masses of molten rock invaded the older rocks from below. The volcanic arc is still active today, decorating the skyline with the cones of Mount Baker and Glacier Peak.

The deep canyons and sharp peaks of today's North Cascades scene are products of profound erosion. Running water has etched out the grain of the range, landslides have softened the abrupt edges, homegrown glaciers have scoured the peaks and high valleys and, during the Ice Age, the Cordilleran Ice Sheet overrode almost all the range and rearranged courses of streams.

Scientists have spent many decades puzzling out some of the geologic history of the range. We invite you to share what we and others have learned at the Geology of the North Cascades website.

Dig deeper

Maps and illustrations

Image gallery

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